In the manufacture of wound capacitors the capacitor electrodes are usually provided by prior metallization of one of the dielectric strip surfaces, but the electrodes may also be provided by a metal strip interposed between non-metallized dielectric strips.
Following completion of winding of the strip, the mandrel is removed from the finished capacitor, which is cylindrical in form, and the latter is then subjected to successive processing phases.
If the strip is wound without adopting special measures, when the capacitor is removed from the mandrel, the initial wound turns of the strip, which constitute the innermost layers, do not maintain the circular form produced by the mandrel. This causes undulation in the initial part of the strip which forms the initial turns, made possible by the axial hole left in the capacitor by the mandrel, giving rise to inward projections: this modifies the characteristics of the capacitor.
The presence of these inward projections may cause considerable problems of a mechanical nature, which have a deleterious effect on the electrical characteristics of the finished product when manufacturing steps which follow winding of the strip are performed, in particular the crushing of the wound capacitor between two planes parallel to its axis to provide a flat sided capacitor.
Crushing, which is intended to flatten the capacitor, eliminating the central hole, can cause accidental bedding down of the projections which are bent over, forming folds, and small quantities of air can remain trapped inside, with detrimental effects on future performance of the capacitor. In addition, folds which fall into a position parallel to the planes between which the capacitor is pressed, cause non-uniform stratification of the rolled strip. During the flattening phase, bosses can be formed on the outer circumference of the capacitors, which are then subjected to increased pressure, and depressions, which are thus subjected to less pressure. Increased pressure in the areas corresponding to the folds, which is difficult to quantify, reduces the thickness of the dielectric strip locally, thus producing substantial reductions in the voltage needed to cause electrical breakdown in such areas. Thus a consistent circular shape of the central aperture of the wound capacitor removed from the mandrel is important for consistent characteristics of the final flattened capacitor.
European Patent No. 0007121 discusses a method for constructing a sleeve in the centre of a wound capacitor, obtained by partial fusion of the initial turns of the wound strip. This is achieved by means of a heated pad which slides on the initial wound turns, according to a preset pressure, transferring heat to the latter. As well as involving a highly complex machine, this method tends to increase the production time for each capacitor. Further, the fusion of the initial turns requires that these turns, if the capacitor is made from metallized dielectric strip, are all demetallized which increases the bulk of small capacitors.